This invention relates to an electrographic apparatus which makes use of a corona charge device and which is constructed to effectively remove detrimental gas such as ozone and dust produced in the corona charge device.
A photosensitive body for electrographic apparatuses, particularly a photosensitive body formed of zinc oxide-resin photosensitive material or selenium or selenium alloy photosensitive material and most frequently used for electrographic apparatuses is charged with the aid of a flow of ions delivered from a corona charge device and can control the flow of ions.
In the conventional corona charge device, a corona discharge wire provides a source of ions, and the electric field produced between the corona discharge wire and a field electrode directs a flow of ions toward a record sheet disposed on the field electrode. Generation of such flow of ions, however, is accompanied by a detrimental gas such as ozone and the like which is liable to deteriorate the charge characteristics of the photosensitive body.
In FIG. 1 is shown a graph which illustrates influence of a detrimental gas such as ozone and the like upon a photosensitive body. A curve "a" shows an electric potential applied to the photosensitive body formed of selenium after it has been subjected to ozone and repeatedly charged and discharged, while a curve "b" shows an electric potential applied to the same photosensitive body after it has been repeatedly charged and discharged during which ozone is removed.
As seen from the curve "b" , if ozone is removed from the photosensitive body, the electric potential applied to the photosensitive body is substantially the same, even after charge and discharge have been repeated for about 40,000 times.
On the contrary, if the photosensitive body is subjected to ozone, the electric potential applied thereto becomes remarkably decayed in darkness as shown by the curve "a".
The reasons why the electric potential on a photosensitive body formed of selenium is remarkably decayed in darkness when it is subjected to ozone are not clearly understood, but it is conceivable that such decay of the electric potential in darkness is caused by crystallization of selenium.
Experimental tests have yielded the result that the photosensitive body formed of zinc-resin photosensitive material shows a more significant decay in darkness of the electric potential applied thereto.
A few attempts have been made to alleviate the influence of detrimental gas such as ozone and the like upon the corona charge device. Such attempts are mainly divided into two measures, one of which is to decrease generation of ozone per se and the other is to remove ozone generated so as not to act upon the photosensitive body.
In FIG. 2 is shown one example of a conventional corona charge device which makes use of the above mentioned latter measure. Two corona discharge wires 1, 1' are connected in common to a negative terminal of a high voltage source 2, the other terminal of which is connected to ground. A shield electrode 3 is connected to ground and a field electrode 4 is also connected to ground. The corona charge device constructed as above described provides a source of ions, and the electric field produced between the corona discharge wires 1, 1' and the field electrode 4 directs a flow of ions toward a record sheet 5 positioned in overlying contact with the grounded field electrode 4. The density of the flow of ions becomes large at the side of the shield electrode 3 if compared with that of the flow of ions at the side of the field electrode 4. As a result, a negative voltage is applied to the shield electrode 3 and ozone generated is attracted to the shield electrode 3 and removed out of the corona charge device.
Such a conventional measure has the disadvantage that a large current flows toward the shield electrode 3, so that current flowing toward photosensitive body 6 becomes decreased thereby degrading the charge efficiency thereof.
In addition, provision may be made of a fan 7 opposed to the shield electrode 3 as shown by dotted lines in FIG. 2 so as to effect a forced attraction of ozone toward the fan 7 to discharge it out of the corona charge device. Such a measure is effective and reliable in operation, but it also has the disadvantage that it is not always easy to construct and arrange the fan 7 such that ozone can effectively be removed from a space extending along the overall length of the corona discharge wires 1, 1'.
A known electrographic apparatus which makes use of a drum type screen photosensitive body has the disadvantage that dust adhered to the photosensitive body 6 is reproduced as an enlarged white point on the record sheet 5, and minute dust deteriorates the charge characteristic of the photosensitive body 6. Experimental tests have also yielded the result that the charge characteristic of the photosensitive body 6 significantly deteriorates when a considerable amount of moisture is present.
In such an electrographic apparatus, the record sheet 5 must be arranged very near the photosensitive body 6 and fed along the latter. In addition, a high voltage electric field is applied between the record sheet 5 and the photosensitive body 6. As a result, if the record sheet 5 makes contact with the photosensitive body 6, the high voltage electric field is short circuited, and hence there is a risk of the photosensitive body 6 being permanently broken. In order to avoid such a short ciruit, provision may be made of a member for urging the record sheet against the field electrode. The use of such a member, however, results in a standstill of the record sheet that will tend to bring the record sheet 5 into contact with the photosensitive body 6.
An object of the invention is to provide an electrographic apparatus provided with an ozone removing device which can obviate the above mentioned disadvantage which has been encountered with the conventional ozone removing device and which is effective and reliable in operation.
Another object of the invention is to provide an electrographic apparatus which prevents the drum type screen photosensitive body from being adhered with minute dust, makes the life of the screen photosensitive body long, provides an improvement in quality of the electrostatic charge image corresponding to an image to be recorded on the record sheet, provides a decrease in deterioration of the charge characteristic of the screen photosensitive body when a considerable amount of moisture is present, prevents a permanent breakage of the screen photosensitive body due to contact with the record sheet, and provides an improvement in quality of the electrostatic charge image formed on the record sheet.